Caesium is a soft, silvery-gold metal that ... two hyperfine levels of the unperturbed ground state of the caesium-133 atom”. Armed with this frequency, calibrated and recalibrated, Nist-F3 ...

The International System of Units then changed things, settling for this definition: "The second… is defined by taking the… transition frequency of the caesium-133 atom, to be 9192631770 when ...

at least — is 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a stationary cesium-133 atom at a temperature of 0K.

The International System of Units then changed things, settling for this definition: The second… is defined by taking the… transition frequency of the caesium-133 atom, to be 9192631770 when expressed ...

The frequency involved is that of the energy absorbed from photons which hit a caesium-133 atom and excite the outermost electron in the atom and cause it to jump from lower to a higher electron ...